I don't know when exactly they started, but I know it was in the 1960s, more particularly during the presidency of John F. Kennedy. An interesting anecdote pertaining to KLM's Chicago operations is that the right to operate to Chicago were first given in person to the then Dutch foreign secretary - and later NATO Secretary General - the late Joseph Luns.

I was recently reading the man's biography, and he describes in this bookwork how during a visit to the Kennedy oval office, he received the traffic rights to fly to Chicago in personal name, and upon return to Holland gave KLM a call to inform the airline that he had received these rights in personal name and whether KLM was interested in those rights as he was not planning in operating them himself anyway. Quite funny.